Monovinylidene aromatic copolymers reinforced with rubber, in particular with diene rubber, represent a well known class of commercially available engineering polymers widely described in the literature. Specific examples of the copolymers are for example styrene and acrylonitrile copolymers, generally referred to as SAN resins, containing rubber particles, for example butadiene, dispersed in the polymeric matrix, generally known as ABS resins.
The rubber-modified monovinylidene aromatic copolymers can be prepared by continuous or batch processes and by various polymerization processes such as bulk, mass-solution, or mass-suspension, these are generally known as mass polymerization processes. A continuous mass polymerization process is known and described for example in U.S. Pat. Nos. 2,694,692; 3,243,481 and 3,658,946, and in published EP 400,479. This process consists of dissolving the rubbery material in the monovinylidene aromatic monomer and ethylenically unsaturated nitrile monomer mixture, adding possibly a radical polymerization initiator and an inert diluent, and then polymerizing the resulting solution. Immediately after the polymerization reaction commences, the rubbery material in the monomer mixture separates into two phases, of which the former, consisting of a solution of the rubber in the monomer mixture, initially forms the continuous phase, whereas the latter, consisting of a solution of the resultant copolymer in the monomer mixture, remains dispersed in form of droplets in said continuous phase. As polymerization and hence conversion proceed the quantity of the latter phase increases at the expense of the former. As soon as the volume of the latter phase equals that of the former, a phase change occurs, generally known as phase inversion.
When this phase inversion tales place, droplets of rubber solution form in the polymer solution. These rubber solution droplets incorporate by themselves small droplets of what has now become the continuous polymer phase. During the process, grafting of the polymer chains on the rubber takes place, too.
Generally, the polymerization is carried out in several stages. In the first polymerization stage, known as prepolymerization, the solution of the rubber in the monomer mixture is polymerized until phase inversion is reached. Polymerization is then continued up to the desired conversion.
Mass polymerization affords rubber-modified monovinylidene aromatic copolymers with a good balance of aesthetic and mechanical properties such as toughness. The good aesthetic properties result, in part, because the mass polymerization process does not produce gels and does not require processing aids, such as emulsifiers, which can impart undesirable color. The good mechanical properties such as toughness result, in part, from the amount and morphology of the rubber. The morphology of the rubber is characterized by particle size and particle phase-volume to rubber ratio. There is an optimal particle size range for optimal toughness, if the rubber particles are too small or too large, toughness decreases. Due to the nature of the mass polymerization process, there are many variables, such as reaction mixture viscosity, rubber levels, rubber types, rubber particle sizing mechanisms, grafting kinetics, coupling kinetics, diluent composition, reactor stirring speed, etc., that control the rubber particle size and morphology. These variables are interrelated and can not be varied independently of each other.
Toughness of articles made from rubber-modified monovinylidene aromatic copolymers also depends upon temperature and deformation rate. Articles with adequate toughness at room temperature often demonstrate inadequate toughness at reduced temperatures. For example see U.S. Pat. No. 6,380,304 which discloses a mass polymerized rubber-modified monovinylidene aromatic copolymer composition with an excellent balance of physical and mechanical properties and high intrinsic gloss at ambient temperature, but makes no mention of good low temperature toughness.
There have been numerous attempts to obtain optimal rubber particle size by controlling the viscosity of the diene rubber used in producing rubber-modified monovinylidene aromatic copolymers, see U.S. Pat No. 4,640,959; EP 277,687; and DE 2,620,853. However, these compositions as well have large rubber particle sizes which are undesirable for toughness.
In view of the deficiencies of the rubber-modified monovinylidene aromatic copolymer compositions thus obtained by utilizing any of such methods it would be highly desirable to provide an economical rubber-modified monovinylidene aromatic copolymer composition which exhibits an improved balance of aesthetic, physical and mechanical properties in particularly good low temperature impact resistance.